NOx in flue gas discharged from a power station or an incinerator is a substance causing photochemical smog and acid rain, and a flue gas denitration method by selective catalytic reduction with ammonia as a reducing agent is used widely as a method for effective removal of NOx mainly in thermal power stations. As the catalyst, a titanium oxide (TiO.sub.2) type catalyst containing vanadium (V), molybdenum (Mo) or tungsten (W) as the active component is used, and particularly the catalyst containing vanadium as one of the active components is highly reactive and can be used at low temperatures (e.g. in a temperature range of 300.degree. C. or less), so it becomes the mainstream of the denitration catalyst at present (Japanese Patent Application Laid-Open No. 50-128681A and the like).
The catalyst in the prior art mentioned above has superior characteristics by which very high degrees of denitration can be achieved in purification of combustion exhaust gases such as gas-fired, oil-fired or coal-fired fuel exhaust gases, but no adequate measures have been taken to purify an exhaust gas from wood waste and wood as fuels abundant in North Europe or the like area and an exhaust gas containing a large amount of deliquescent salts in ash from an incinerator and the like, and there has been the problem that the degrees of denitration are lowered with time.
FIG. 7 shows the degrees of denitration and the change of the amount of an alkali accumulated in the conventional denitration catalyst in the case (A) where the conventional catalyst was exposed to a wood-fired boiler combustion exhaust gas as an example of application to an exhaust gas containing a large amount of deliquescent salts and the case (B) where the catalyst was exposed to a coal-fired boiler exhaust gas as an example not containing deliquescent salts. In the case (A) of the wood-fired boiler containing a large amount of potassium carbonate as a deliquescent alkali metal salt, there occurs the phenomenon in which the amount of the alkali in the catalyst is increased and the degree of denitration rapidly decreases.
Such deterioration of the denitration catalyst by the alkali metal salt also occurs where an incinerator exhaust gas or a high-sulfur oil-fired boiler exhaust gas is to be denitrated, thus greatly preventing practical application of the low-temperature denitration method to such exhaust gases.
It was found that the above deterioration by the highly deliquescent salt is caused by inclusion, in ash, of (1) potassium carbonate in the case where wood waste is used as the fuel, (2) calcium chloride or sodium chloride in the case of refuse combustion exhaust gas, and (3) sodium sulfate and potassium sulfate in the case of high-sulfur oil-fired boilers. Accordingly, it is believed that although the alkali metal salts in ash differ depending on the type of exhaust gas as described above, any deterioration of the catalyst during low-temperature denitration is caused by the common mechanism in which the alkali metals and alkaline earth metal salts contained in ash absorb moisture to deliquesce to form a liquid in the step of temperature raising when a denitration apparatus starts or stops, so that the fluid of the metal salts penetrates into the catalyst to cause the clogging of pores therein and to deteriorate the active site.
To prevent the deterioration of the denitration catalyst by such fluid having deliquesced, it is necessary to physically prevent the fluid that has deliquesced from entering the denitration catalyst.
The present invention provides a highly durable denitration catalyst which is prevented from undergoing rapid deterioration upon deliquescence of salts, which deterioration occurs in the case of denitration of an exhaust gas containing ash having a wide variety of deliquescent salts as described above, a process for preparing the the denitration catalyst, and a method for purification of an exhaust gas by use of the same.